The present invention relates to the art of heat dissipation from high power semiconductor devices. The invention finds particular application in conjunction with power amplifiers configured to power gradient coils within a magnetic resonance imaging apparatus and will be described with particular reference thereto. The invention will also find application in conjunction with the manufacture, an arrangement of power semiconductors such as field effect transistors (FET) where good thermal contact between the semiconductor package and a heat sink is desired.
Power semiconductors come in many packages with a common problem. That problem is making good thermal contact with a heat sink so that the power semiconductors can transfer excess thermal energy and continue to operate efficiently. This effective thermal contact can be accomplished by applying pressure against the power semiconductor toward a flat surface of a heat sink. One known way thought to provide good thermal contact between the semiconductor and the heat sink is to bolt the semiconductor directly to the heat sink through a hole or channel in the package. This method applies pressure most intensively near the channel, however clamping pressure decreases as distance from the channel increases. Indeed, depending on a channel directly through the semiconductor is also problematic in that the presence of this channel is not universal among semiconductors of different ratings and from different manufacturers.
Another known method of attempting to ensure positive thermal contact between a semiconductor and a heat sink involves a clamping plate over the semiconductor to apply pressure to the top of the package. Unfortunately, when more than one package is clamped, as is desirable in gradient coil amplifiers, uneven pressure is applied by the clamping member over the thermal contact area. Additionally, small but significant variations in package tolerances, thermal expansion, and deformations in the heat sink and clamp contribute to a non-uniform pressure supplied by the plate to the several semiconductor packages present.
The present invention contemplates improved apparatus for clamping power semiconductors to a heat sink which overcomes the above-referenced problems and others.
In accordance with one embodiment of the present invention, a magnetic resonance imaging system includes a main magnetic field generator producing a temporally constant magnetic field through an imaging region. A gradient coil assembly is also provided and powered by a gradient field amplifier. The gradient field amplifier includes a heat sink for dissipating thermal energy generated by at least one power semiconductor device such as a field affect transistor. The semiconductor device includes a first surface opposite a thermally conductive surface which is in thermal contact with the heat sink. A rigid plate clamp mechanically connects to the heat sink and the semiconductor device is positioned therebetween. A resiliently deformable spring is disposed between the first surface of the power semiconductor and the plate clamp. The spring maintains positive mechanical pressure on the semiconductor toward the heat sink.
In accordance with another aspect of the present invention, the deformable spring comprises an annular element defining an inner ring and an outer ring. The inner ring is in contact with the rigid plate while the outer ring makes contact with the first surface of the semiconductor device.
In accordance with another aspect of the present invention, the deformable spring consists of a spring disc washer.
In accordance with another aspect of the present invention, a gradient field amplifier includes a rigid plate clamp removably affixed to a heat sink. At least one transistor package is contained between the heat sink and the rigid plate clamp. The transistor package includes a thermally conductive surface in thermal contact with the heat sink for dissipating thermal energy from the transistor. The resilient spring washer is mechanically engaged between the rigid plate clamp in the transistor package urging the thermally conductive surface of the package toward the heat sink.
In accordance with another aspect of the present invention, a diameter of the resilient spring washer corresponds substantially to that of the transistor package.
In accordance with another embodiment of the present invention, the power semiconductor device includes a package having a first surface opposite a thermally conductive surface. A resiliently deformable spring is affixed to the first surface of the package for maintaining positive mechanical pressure between the thermally conductive surface of the semiconductive surface of the semiconductor and the heat sink.
One advantage of the present invention resides in enhanced clamp pressure for transistors onto a heat sink.
Another advantage of the present invention resides in the ability to clamp multiple grouped transistors with a single plate while maintaining individual clamp pressure to assure maximum heat transfer.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.